Machines and facilities used at many production sites are typically controlled by a control system including a control device configured by a programmable logic controller (hereinafter, also abbreviated as a “PLC”) or the like.
The PLC and a single or a plurality of remote 10 terminals sometimes communicate, with the PLC functioning as a master device configured to control the entire communication, in accordance with the polling method. For example, JP 2007-312043 A (Patent Literature 1) discloses two typical types of a communication method, namely, the broadcast method and the polling method, for communication between a master device and a slave device in a remote 10 system.
A recent industrial automation site typically has a network (also called a fieldbus) built up for transmission of control commands and data signals. Examples of such a network include the EtherCAT (registered trademark). The EtherCAT is a fieldbus featuring synchrony and configured to achieve synchronization among all slave devices with jitter of 1 μs or less. Each of the slave devices receives an output signal (control signal) from a master device at such synchronization timing and causes an externally acquired value to be reflected to an input signal (a measured value, a status signal, or the like) transmitted to the master device.
Meanwhile, there is an optical measurement apparatus configured to execute continuous measurement with internal synchronization. The EtherCAT connected with such an optical measurement apparatus has an issue of synchronization among devices connected to the EtherCAT. Achievement of highly accurate measurement depends on when to acquire data reflected to a signal transmitted from each of the devices to a PLC.
However, the optical measurement apparatus typically needs to receive light for a certain period of time to acquire a measured value. A measurement period thus includes a light receiving period. There is further needed a period for generation of a measured value from light receipt data in the optical measurement apparatus. The optical measurement apparatus thus occasionally has measurement timing not in synchronization with measurement result output timing. It is thus difficult for a master device to determine measurement timing of a result reflected to data outputted from the optical measurement apparatus.